Official Luthiers Forum!

in looking at many picts of X braced guitars I see a huge variation. Asuming
the same scale length and placement of the X brace midpoint, their can be
may angles used.
It seems most Martins are around 95 degrees. Some use 90 degrees
The R Taylors are around 100 and the picts in the Wayne Henderson book
look like at least 105.
What are the pros and cons of opening and closing up the X brace angles?
TIA

_________________
Andy Z.
http://www.lazydogguitars.com

In simplest terms, A very tall X would make the soundboard stiffer longitudinally, while a very short x would give great cross-grain stiffness... Think about how that would shape the sound.

One way to look at it is to think about how much stiffness you are adding along and across the grain. Most top wood is much stiffer along the grain, of course, and if the objective is to get the lengthwise and crosswise stiffness in some sense 'the same', then the bracing has to run across the top at some angle. The stiffness ratio of different pieces of top wood varies a lot, ranging from something like 8:1 to maybe 15:1 or more, depending. Thus, all else equal, you might want to change the X angle to compensate for differences in cross grain stiffness in the top.

The shape of the guitar will also effect this, of course. A top that is relatively narrower won't need as much crosswise stiffness added in from the bracing, so you'd probably use a different X angle for a 00 than a Jumbo.

Finally, a lot will depend on what you do with the tone bars and finger braces. It's not uncommon for the tone bars to run more 'across' the top than the X braces, and that would call for a narrower X angle, all else equal.

In the end, it's something you have to work out. The standard designs are set up to work pretty well with average wood, and won't be too far off with wood that's very stiff or very floppy across the grain. They are compromises, and there's a range of values that will work pretty welll. If you keep track of the stiffness ratio of the woods you use it should help you figure out what brace angle will work best for you in given circumstances.

Thanks

_________________
Andy Z.
http://www.lazydogguitars.com

If you think in terms of monopole, long dipole, and cross dipole then changing the shape of the x should have predictable results (within reason).

Another thing that I find interesting is the body shape. If you look at where the braces are in relationship to the waist - on tight waisted guitars they are VERY stiff in this area due to the fact that the braces run nearly to the edges of the sides vs. on wider waisted guitars the braces don't come as close and thereby don't firm up the top as much.

That changes the dynamics of the top dramatically and explains a lot with respect to why dreads and GA's sound so different.

_________________
Brock Poling
Columbus, Ohio
http://www.polingguitars.com

Thanks
I guess I will learn a lot more about monopole dipole and cross dipole at
Ervins

_________________
Andy Z.
http://www.lazydogguitars.com

That's a good question Andy. I've been wondering the same thing. Now I just do what looks right to me so it probably is wrong! Let us know if you reach any conclusiois wrong

_________________
Joe Beaver
Maker of Sawdust